This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 99-21855 filed in Korea on Jun. 11, 1999, 99-22419 filed in Korea on Jun. 15, 1999, and 00-30352 filed in Korea on Jun. 2, 2000; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a synthesis method of carbon nanotubes, and more particularly, to a mass synthesis method of synthesizing high purity carbon nanotubes vertically aligned over a large area substrate.
2. Description of the Related Art
Carbon nanotubes, which have conductivity in an arm-chair structure and semiconductivity in a zig-zag structure, are applied as an electron emission source for field emission devices, white light sources, lithium secondary batteries, hydrogen storage cells, transistors or cathode ray tubes (CRTs). For such industrial applications of carbon nanotubes, it is profitable to synthesize high purity carbon nanotubes over a large-area substrate in a vertically aligned form. Also, it is another concern that the diameter and length of carbon nanotubes, and the density and uniformity of carbon nanotubes over a substrate used can be easily controlled for the carbon nanotube synthesis.
Existing carbon nanotube synthesis techniques include an arc discharge method, laser vaporization method, gas phase synthesis, thermal chemical vapor deposition (CVD) method, plasma CVD method and the like.
The arc discharge method (C. Journet et al., Nature, 388, 756 (1997) and D. S. Bethune et al., Nature, 363, 605 (1993)) and the laser vaporization method (R. E. Smally et al., Science, 273, 483 (1996)) are not able to control the diameter or length of carbon nanotubes and the yield by these methods is low. Moreover, excess amorphous carbon lumps are also produced along with carbon nanotubes, and thus they need complicated purification processes. Thus, it has a difficulty in growing carbon nanotubes over a large-size substrate on a large production scale by these methods.
Meanwhile, the gas phase synthesis method (R. Andrews et al., Chem. Phys. Lett., 303, 468, 1999), which is appropriate for mass synthesis of carbon nanotubes, produces carbon nanotubes in a gas phase by pyrolysis of carbon source gas in a furnace without using a substrate. However, this method also has difficulty in controlling the diameter or length of carbon nanotubes, and causes adhering of metal catalyst lumps to the inner or outer sidewalls of carbon nanotubes. Thus, the method cannot meet the need for high purity carbon nanotubes and cannot achieve vertical alignment of carbon nanotubes over a substrate.
The thermal CVD method known in this art up to now involves growing carbon nanotubes over a porous silica (W. Z. Li et al., Science, 274, 1701 (1996)) or zeolite (Shinohara et al., Japanese J. of Appl. Phys., 37, 1357 (1998)) substrate. However, filling pores of the substrate with a metal catalyst is a complicated and time consuming process. Moreover, the controlling of the diameter of carbon nanotubes is not easy, and the yield is low. Thus, the thermal CVD method has a limitation in growing massive carbon nanotubes over a relatively large substrate.
The plasma CVD method (Z. F. Ren et al., Science, 282,1105 (1998)) is a suitable technique for vertically aligning carbon nanotubes, with excellent performance. However, there are problems in that plasma energy damages carbon nanotubes and the structure of the carbon nanotubes is unstable due to the synthesis process at low temperatures. In addition, many carbon particles adhere to the surface of carbon nanotubes.
To solve the above problems, it is an objective of the present invention to provide a mass synthesis method of high purity carbon nanotubes vertically aligned over a large-size substrate.
The objective of the present invention is achieved by a method of synthesizing carbon nanotubes, comprising forming a metal catalyst layer over a substrate. The metal catalyst layer is etched to form isolated nano-sized catalytic metal particles, and carbon nanotubes vertically aligned over the substrate are grown from respective isolated nano-sized catalytic metal particle by thermal chemical vapor deposition (CVD) in which a carbon source gas is supplied to a thermal CVD apparatus to form carbon nanotubes.
Preferably, forming the isolated nano-sized catalytic metal particles is performed by a gas etching method in which one etching gas selected from the group consisting of ammonia gas, hydrogen gas and hydride gas is thermally decomposed for use in etching. Forming the isolated nano-sized catalytic metal particles may be performed by plasma etching, or wet etching using a hydrogen fluoride series etchant.
Preferably, the etching gas is ammonia, and the gas etching method is performed at a temperature of 700 to 1000xc2x0 C. while supplying the ammonia gas at a flow rate of 80 to 400 sccm for 10 to 30 minutes.
Preferably, forming the carbon nanotubes is performed at a temperature of 700 to 1000xc2x0 C. while supplying the carbon source gas at a flow rate of 20 to 200 sccm for 10 to 60 minutes.
Preferably, forming the catalytic metal particles and forming the carbon nanotubes are in-situ performed in the same thermal CVD apparatus.
Preferably, in forming the carbon nanotubes, one gas selected from the group consisting of ammonia gas, hydrogen gas and hydride gas is supplied to the thermal CVD apparatus along with the carbon source gas.
Preferably, after forming the carbon nanotubes, the synthesis method further comprises exhausting the carbon source gas using an inert gas from the thermal CVD apparatus.
Preferably, after forming the carbon nanotubes, the synthesis method further comprises in-situ purifying the carbon nanotubes in the same thermal CVD apparatus. Preferably, in-situ purifying the carbon nanotubes is performed with a purification gas selected from the group consisting of ammonia gas, hydrogen gas, oxygen gas and a mixture of these gases.
Preferably, after in-situ purifying the carbon nanotubes, the synthesis method further comprises exhausting the purification gas using an inert gas from the thermal CVD apparatus.